Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 201743

Area Forecast Discussion
National Weather Service Cheyenne WY
1143 AM MDT FRI MAY 20 2016

Issued at 1124 AM MDT Fri May 20 2016

showers and thunderstorms will develop by early this afternoon
along the laramie range. This activity will spread northeast into
the plains during the afternoon hours. There is the potential for
a few marginally severe thunderstorms. effective shear is on the
lower end of the super cell thunderstorm spectrum between 20 and
30 kts. MU capes should mainly be between 500-1500 j/kg across the
plains this afternoon. The nam is showing near 2000 j/kg around
Scottsbluff at 21z. based on shear and cape a few severe
thunderstorms are certainly possible...especially as storms move
into the lower north platte river valley as there is more low
level moisture for storms to tap into. The main severe
thunderstorm threat will be hail with gusty winds also possible.
the threat for tornadic activity is very low.


.SHORT TERM...(Today through Sunday)
Issued at 330 AM MDT Fri May 20 2016

Overnight water vapor imagery depicted an active large scale
pattern across the Lower 48 with one upper low departing northern
New England...a second approaching the mid-MS valley...and the
third over the northwest CONUS. The surface analysis placed a
front from eastern MT across central WY...northwest UT...southern
NV and CA. A weak shortwave moving across western SD and northwest
NE was producing widely scattered showers and thunderstorms. This
convective activity will exit the northern NE panhandle by daybreak.
Otherwise...skies were partly cloudy and temperatures were in the
40s to lower 50s. Prevailing winds were southerly 5-15 kt.

00z short range models are in good agreement with the amplification
of the Great Plains upper ridge and western CONUS upper low. The
eastward progression of these systems will be slow through the
short term. Lift from a series of shortwaves rounding the base of
the trough and ejecting north-northeast across the region will
enhance convective development...especially during the afternoons
and evenings. The front to the west will remain nearly stationary
until Sunday when the upper low lifts northeast into MT...inducing
eastward motion across the CWA. Today and Saturday will experience
above normal temperatures with the low/mid level thermal ridge
over the high plains. 700mb temperatures rise into the lower teens
Celsius both days...yielding max temperatures into the 70s to near
80s for the valleys and plains. Instability will once again be
focused initially over the high terrain early this afternoon with
convection developing. Widely scattered thunderstorms will push
out onto the eastern WY plains mid to late this afternoon. SPC has
highlighted the risk for marginally severe thunderstorms over the
high plains...along and east of I-25. The convection may extend
as far east as the northern NE panhandle...but will have a
difficult time holding together further south as the atmosphere
will be capped. The main threats will be hail up to nickel size
and gusty winds to 45 mph.

Saturday may be a different story as a shortwave ejecting across
the high plains interacts with CAPEs of 1000-1500 j/kg and 30 kt
low level shear. The SPC Day 2 outlook indicates a risk for
marginally severe thunderstorms from east central WY across
western NE with isolated large hail...wind gusts to 58 kt and
brief heavy rain. The pressure gradient will increase today with
gusty southerly winds...especially west of the Laramie Range.
Gusts up to 35 kt are expected...with 20-25 kt elsewhere. Similar
winds are forecast Saturday. Sunday will trend cooler with the
passage of the cold front. Temperatures west of the Laramie Range
will be 10-15 degrees cooler...while areas east of I-25 will not
feel the impact of the cooler temperatures until later in the day.

.LONG TERM...(Sunday night through Thursday)
Issued at 245 AM MDT Fri May 20 2016

Sunday night...Expect isolated to widely scattered coverage of
strong thunderstorms in the evening mainly east of a Lusk to
Cheyenne line along and east of a low level convergence boundary,
then decreasing coverage of showers and thunderstorms after midnight.

Monday...Low and mid levels dry out somewhat, thus expect a
corresponding decrease in afternoon and evening thunderstorm coverage,
though isolated thunderstorms anticipated south of a Laramie to
Alliance line.

Tuesday...Flow aloft remains southwest. With a slight increase in
low and mid level moisture, also expect a similar increase in
afternoon and evening thunderstorm coverage.

Wednesday...Southwest flow aloft continues. Low level convergence
boundary progged to be across our southeast counties, where isolated
to scattered afternoon and evening thunderstorms will be focused.

Thursday...Weak troughiness aloft will lie across our counties,
and with enough low and mid level moisture, expect isolated to
scattered afternoon and evening showers and thunderstorms, mainly
along and east of Interstate 25.


.AVIATION...(For the 12Z TAFS through 12Z Saturday morning)
Issued at 530 AM MDT Fri May 20 2016

Localized IFR at Alliance and Sidney through 15z today in fog and
low clouds. Isolated thunderstorms at Laramie and Cheyenne from
20Z to 02Z. Otherwise, VFR prevails. Winds gusting 20 to 33 knots
from late this morning until early this evening.


Issued at 245 AM MDT Fri May 20 2016

No fire weather concerns with green fuels and ample surface moisture.
Warmer than normal temperatures will prevail today and Saturday...
followed by cooler temperatures with a cold frontal passage
Sunday. Isolated to scattered showers and thunderstorms can be
expected...mainly during the afternoons and evenings through the
weekend and much of next week. Southerly winds will be gusty over
the high plains and especially west of the Laramie Range. Minimum
humidities will be above critical thresholds.


Issued at 1124 AM MDT Fri May 20 2016

Showers and thunderstorms will mainly occur along and east of the
Laramie range today. There will generally be little impact on
rivers from showers and thunderstorms today but locally heavy rain
will be possible through the lower north platte river valley this

Streams in the mountains have thus far been slow to respond to
warmer temperatures. Should see a little more response tonight
through saturday as the warmest air so far this year moves across
the area and wind speeds increase somewhat.


.CYS Watches/Warnings/Advisories...



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